Membrane fluidity, as probed by the lateral diffusive motions of photopigment on vesicular surface of disc membranes of bovine retina, is examined in reference to the photoreceptor function. Specific tools employed are the laser light scattering technique in the forward-depolarized configuration, the electrophoretic scattering, and the fast rise optical photometry. Having established the deformation characteristics of these membranes by osmotic pressure and photobleaching, the dynamics of lateral diffusion of rhodopsin (or opsin) by laser scattering constitutes unique, non-perturbing fluidity probe.